Infections due to Mycobacterium tuberculosis (Mtb) are the leading causes of morbidity and mortality in both HIV infected and immune competent people. The hallmark of tuberculosis is latency, where bacteria are presumed to be metabolically active, but in a state of persistence. Cell division is a critical aspect of cell cycle, without which pathogen multiplication and subsequent infection does not ensue, and hence is expected to be regulated during 'entry into'and 'exit from'the persistent state. The genetic and biochemical aspects of cell division and the mechanisms used to regulate this process in Mtb at different stages of growth are largely unknown. The cell division process is believed to be mediated by several proteins that localize to the division site, and form a multiprotein complex called the septal ring. The initial step in the septal ring assembly appears to be the polymerization of a highly conserved tubulin-like protein, called FtsZ, in the form of a ring (Z-ring). Our recent studies indicate that Mtb growing in macrophages are filamentous and deficient for midcell Z-rings implying that Z-ring assembly and cell division are subject to regulation in vivo. One mode of regulation appears to involve targeting of a novel cell-wall hydrolase (CWH) activity encoded by Rv2719c (chiZ) to potential Z-ring assembly sites. Our proposal aims to investigate how Z-ring assembly and cell division are regulated during growth in vivo. We hypothesize that the intracellular milieu alters the expression and activities of unidentified FtsZ interaction partners and thereby modulates midcell Z-ring assembly and cell division. Using genetic, biochemical and cell biological approaches we propose to identify both the specific in vivo environment and the potential FtsZ interaction partners modulating midcell Z-ring assembly during intracellular growth. In vivo and in vitro interactions between putative regulators and FtsZ will be investigated in an effort to understand how Z-ring assembly is altered in response to infection. The biochemical activities and the potential mechanism of action of ChiZ in regulating Z-ring assembly, and therefore cell division, will be investigated. It is hoped that these experiments will define how the cell division process in Mtb is regulated and will advance our understanding of proliferation of Mtb in vivo.